This invention relates generally to automated-guided vehicle (AGV) systems and, in particular, to navigation and control systems for guiding an automated-guided vehicle along a system guide path. The invention finds application in material handling, such as movement of material within a factory, as well as on-road and off-road vehicles.
Automated-guided vehicles have become extremely effective at movement of materials between processes in a manufacturing plant. Each of a plurality of AGVs automatically carries a load from a pickup point to a discharge point along a system guide path. Navigation of the AGV is typically either by reference to fixed guides, such as guide wires positioned in the floor along the guide path, or by dead-reckoning. Dead-reckoning systems utilize sensors within the AGV in order to monitor the heading, rate-of-change of heading and distance traveled by the AGV along its longitudinal axis which is controlled to coincide with the guide path. The dead-reckoning systems are advantageous because they avoid the enormous expense of placing guide wires in the floor along the entire guide path. Additionally, such dead-reckoning systems are flexible because the guide path layout may be altered by programming changes in the controls rather than requiring tearing and repositioning of the guide wires.
Dead-reckoning systems rely upon an integration of the rate of turn of the vehicle and the distance traveled to maintain position information of the vehicle. Because such measurements tend to drift with time, it is known to supplement the dead-reckoning navigation system with a location verification system, such as markers positioned at known locations along the system guide path. These markers are sensed by a sensor assembly on the AGV as the AGV moves along the guide path in order to verify and compensate, if necessary, the position of the AGV.
One type of marker is a cylindrical magnet positioned in the floor which is sensed by a sensor assembly made up of a series of magnetic sensors, such as Hall-effect sensors, laterally spaced along the bottom of the AGV body. Every time the vehicle passes over a magnet, a location of the body with respect to the magnet is determined from the outputs of the magnetic sensors and used to update the position information of the vehicle. An example of such a system is disclosed in U.S. Pat. No. 4,772,832 to Okazaki et al. Such a system is also utilized in automated-guided vehicle systems marketed by Applicants"" assignee, Rapistan Systems of Grand Rapids, Mich. The Rapistan Systems AGV is marketed under various model numbers, such as Model No. DT-100,and is embodied in various forms. These include tuggers and unit load carriers, to name a few. The sensor assembly employed in the Rapistan Systems AGV includes a series of magnetic sensors in the form of Hall-effect sensors spaced approximately 1 inch apart. The position of the vehicle with respect to a magnet is determined by identifying the three sensors having the highest output and interpolating the values of the outputs to identify the maximum magnetic field intensity sensed by the sensor assembly. While the theoretical accuracy of such system is within xc2xc inch, there is a tendency for the existing algorithm solution to group, or settle, at the integer distance values, i.e., every inch, which corresponds to the locations of the magnetic sensors. This tendency to settle on the integers reduces the accuracy of the detection of the magnet.
Another difficulty with the known magnet-based position update system is that the magnet is positioned with one of its opposite poles directed upwardly so that the magnetic field sensed by the sensor assembly is always unipolar and of the same polarity. While this makes identification of the maximum magnetic field strength easier, the system is unable to determine the direction that the vehicle body passes over each magnet. Information on the direction that the vehicle passes each magnet could be helpful to the system.
For example, each vehicle must be initialized into the system at an initialization station. An initialization station consists of two spaced apart magnets at a unique distance. The initialization station provides initial position and bearing information to the vehicle dead-reckoning system to allow the vehicle to travel along the guide path. Because it is necessary that each initialization station be uniquely identified to the vehicle, various schemes have been proposed to provide that information to the vehicle. One scheme is to vary the spacing between the magnets at the initialization station. For example, the two magnets may be spaced apart at unique separation distances which, for example, may be between 4 feet and 15 feet. In order to accommodate tolerances, the unique separation distances are provided in steps, such as 6-inch steps, in order to ensure that one initialization station is not mistaken for another. The requirement that the separation of the magnets be uniquely assigned in no finer than 6-inch increments and between fixed limits, such as 4 feet and 15 feet, severely limits the number of initialization stations in the system. In this example, no more than 22 stations are possible. This, in turn, limits the flexibility in constructing very large systems.
Furthermore, with the known initialization station, it is possible to erroneously initialize the vehicle traveling in the wrong direction across the two magnets. This may occur even though an arrow may be inscribed on the floor adjacent the magnets. When a vehicle is initialized in this manner, operation of the vehicle is erroneous.
Proposals have been made for magnetic position update systems that present magnetic fields of opposite polarity, generated by the north and south poles of the magnet at the surface of the pathway, for sensing by the sensor assembly. One such system is disclosed in U.S. Pat. No. 4,908,557 issued to Sudare et al. In Sudare et al., a position of the vehicle with respect to each magnet is determined by repeatedly scanning the magnetic sensors as the vehicle passes over the magnet in order to determine a region pair which includes two regions of equal level of magnetic magnitude. A shortest distance is obtained with respect to the two regions and a center position of the two regions is selected on the basis of the shortest distance. This is supposed to represent a null point between the opposite polarity fields. While the Sudare et al. system provides the ability to determine the direction that the vehicle passes over each magnet, it is not without its difficulties. The Sudare et al. system has precision of measurement on the order of magnitude of center-to-center spacing of the Hall-effect sensors.
One of the difficulties in sensing a position of an AGV body with respect to a magnet assembly producing a bipolar magnetic field is that the maximum strength of the associated polarity of the field does not tend to correspond with any physical place on the magnet, such as the north face or south face. In order to be able to determine a relative position between an AGV body and a magnet assembly, a predetermined position on the magnet assembly must be selected for sensing. Because the maximum field strength for a polarity does not necessarily correspond with a location on the magnet generating the bipolar magnetic field with respect to the surface the vehicle travels, it becomes difficult to locate the magnet assembly in the coordinates of the factory floor. Another difficulty with sensing the bipolar magnetic field is that, unlike a unipolar magnetic field, the axis passing through the north and south poles can become skewed with respect to the guide path unless expensive placement techniques are used. Such skewing could significantly affect the ability to establish a unique position of the magnet assembly with respect to the vehicle body.
The present invention provides an automated-guided vehicle system in which position updates are obtained from magnets generating bipolar magnetic signals in which accuracy of the relative position between the vehicle and the magnet assembly is significantly better than that achieved by the prior art. By utilizing magnet assemblies capable of producing bipolar magnetic signals, the present invention provides information on the direction being traveled by the vehicle as it traverses a magnet assembly. This may be useful in providing an improved initialization station as well as other enhanced features in an AGV system.
The present invention also provides data on absolute magnitude of the sensed magnetic field generated by the magnet assembly. This information may be useful in providing a diagnostic system whereby successive readings obtained at each magnet assembly may be compared in order to monitor the operation of each magnet assembly. The successive readings may be taken by the same vehicle every time it passes over a particular magnet assembly, or by multiple vehicles traversing the same magnet assembly. When successive readings taken at the same magnet assembly indicate variation in sensed absolute magnitude of the magnetic field, or variation from one magnet assembly to the other magnet assemblies, a diagnosis may indicate a defective magnet assembly or a change in its operating environment.
An automatic-guided vehicle system, according to an aspect of the invention, includes at least one automated-guided vehicle and pathway for the at least one automated-guided vehicle. The at least one automated-guided vehicle includes a body, a plurality of wheels for transporting the body across a surface, a navigation and guidance system and a sensor assembly. The pathway is defined by a surface and includes a plurality of magnet assemblies positioned along the surface. Each of the magnet assemblies includes at least one magnet defining a pair of opposite magnet poles. The sensor assembly is made up of a plurality of magnetic sensors extending generally transverse to the pathway. The sensor assembly is positioned at the body for sensing each magnet assembly as the body is transported over that magnet assembly by its wheels. The sensor assembly produces an output indicative of intensity of respective portions of the magnetic field sensed by the magnetic sensors. The navigation and guidance system receives the output from the sensor assembly and determines from the output a location of maximum magnitude of the magnetic field sensed by the sensor assembly at each magnet assembly. The navigation and guidance system determines a position of the body with respect to a magnet assembly as the sensor assembly passes over that magnet assembly from the maximum magnitude of the magnetic field. According to this aspect of the invention, the navigation and guidance system determines the location of the substantially maximum magnitude of the magnetic field by mathematically fitting a curve to the intensity of portions of the magnetic field sensed by the magnetic sensors and determining a maximum value of the curve.
According to another aspect of the invention, an automated-guided vehicle system includes at least one automated-guided vehicle and a pathway for the at least one automated-guided vehicle as previously set forth. According to this aspect of the invention, at least some of the magnet assemblies each produce a bipolar magnetic field at the surface extending generally transverse to the pathway with respect to movement of the at least one automated-guided vehicle along the pathway. The sensor assembly is made up of magnetic sensors extending generally transverse to the pathway. The sensor assembly is positioned at the body for sensing the bipolar magnetic field of each of the magnet assemblies as the body is transported over the magnet assembly by the wheels. The sensor assembly produces an output indicative of the magnitude and polarity of respective portions of the magnet field sensed by the magnetic sensors. The navigation and guidance system receives the output from the sensor assembly and determines from the output a location of the substantially maximum magnitude of at least one polarity of the bipolar magnetic field sensed by the sensor assembly at each magnet assembly. The navigation and guidance system determines a position of the body with respect to the magnet assembly as the sensor assembly passes over that magnet assembly from the substantially maximum magnitude of the at least one polarity of the bipolar magnetic field sensed by the sensor assembly. According to this aspect of the invention, the navigation and guidance system determines a correction factor for correcting offset between the substantially maximum magnitude of the at least one polarity and predetermined location on that magnet assembly, including correcting for any skew of the bipolar magnetic field with respect to the pathway for the magnet assembly. The navigation and guidance system corrects the position of the body with respect to a magnet assembly with the correction factor for that magnet assembly.
A method of guiding an automated-guided vehicle, according to an aspect of the invention, includes providing at least one automated-guided vehicle and a pathway for the at least one automated-guided vehicle. The pathway is defined by a surface and includes a plurality of magnet assemblies positioned along the surface. The at least one automated-guided vehicle includes a body and a sensor assembly made up of a plurality of magnetic sensors extending generally transverse to the pathway. A method according to this aspect of the invention includes sensing each magnet assembly with the sensor assembly as the body is transported over the magnet assembly thereby producing an output indicative of intensity of respective portions of the magnetic field sensed by the magnetic sensors. The method further includes determining a location of the substantially maximum magnitude of the magnetic field sensed by the sensor assembly at each magnet assembly and determining a position of the body with respect to a magnet assembly as that sensor assembly passes over that magnet from the substantially maximum magnitude of the magnetic field. The method further includes determining the location of the substantially maximum magnitude of the magnetic field by mathematically fitting a curve to the intensity of portions of magnetic fields sensed by the magnetic sensors and determining a maximum value of the curve.
By determining a maximum value of the curve corresponding to the substantially maximum magnitude of the magnetic field, a database can be established of readings from multiple passes across each of the magnet assemblies. In this manner, diagnosis of the condition of each magnet assembly can be made and maintained in order to determine when maintenance is required on any particular magnet assembly or on an automated-guided vehicle. Furthermore, determining of a maximum value of a curve mathematically fitted to the intensity of portions of magnetic fields sensed by the magnetic sensors provides an exceptionally accurate value of maximum magnitude of the magnetic field sensed by the sensor assembly. In this manner, a more accurate determination of a position of the AGV body with respect to the magnet assembly as the sensor assembly passes over that magnet assembly can be made, thereby improving the overall accuracy of the guidance of the AGVs.
A method of guiding an automated-guided vehicle according to yet another aspect of the invention includes providing at least one automated-guided vehicle and a pathway for the at least one automated-guided vehicle. The pathway is defined by a surface and includes a plurality of magnet assemblies positioned along the surface. At least some of the magnet assemblies includes at least one magnet defining a pair of opposite magnetic poles producing a bipolar magnetic field at the surface extending generally transverse to the pathway with respect to movement of the automated-guided vehicle along the pathway. The at least one automated-guided vehicle includes a body and sensor assembly made up of a plurality of magnetic sensors extending generally transverse to the pathway. The sensor assembly produces an output indicative of the magnitude and polarity of respective portions of the bipolar magnetic field sensed by the magnetic sensors. The method, according to this aspect of the invention, includes receiving outputs from the sensor assembly and determining from the outputs a location of substantially maximum magnitude of at least one polarity of the bipolar magnetic field and determining a position of the body with respect to a magnet assembly as the sensor assembly passes over that magnet assembly from the substantially maximum magnitude of the at least one polarity of the bipolar magnetic field sensed by the sensor assembly. The method further includes determining a correction factor for correcting offset between the substantially maximum magnitude of the at least one polarity and a predetermined location on that magnet assembly including correcting for any skew of the bipolar magnetic field with respect to the pathway for that magnet assembly. The method further includes correcting the position of the body with respect to the magnet assembly with the correction factor for that magnet assembly.
By providing the ability to accurately determine the location of the AGV body with respect to a magnet assembly producing a bipolar magnetic field transversed to the pathway, the present invention, for the first time, facilitates the use of magnet assemblies producing such bipolar magnetic fields without detriment to the position updating function. This is accomplished by determining a correction factor. The correction factor allows a precise determination of a location of the vehicle with respect to a location, such as the north face, or south face of the magnet assembly. This is in contrast to prior art magnet assemblies positioned with their bipolar magnetic field transverse the pathway, wherein the vehicle attempts to identify the null between the opposite polarity fields. Furthermore, the correction factor corrects for any skew of the bipolar magnetic field thereby reducing the necessity for accurately positioning the magnets with respect to the pathway.
By facilitating the use of magnets producing a bipolar magnetic field transverse the pathway, the present invention provides an indication to the vehicle of the direction the vehicle is travelling with respect to the magnet. This provides the ability to, for the first time, code the magnets defining an initialization station in order to increase the number of uniquely identifiable initialization stations in the system. In addition, it is possible to determine that an automated-guided vehicle has traversed the initialization station in an incorrect direction thereby indicating the need to perform the initialization function over again.
Furthermore, the use of a magnet assembly producing a bipolar magnetic field transverse the pathway allows the magnet assembly to function as a signaling device signaling information to the vehicle. For example, one pole of the bipolar magnetic field may be an electromagnet that is controllable as part of an overall control scheme. By selectively activating that pole, the magnet assembly may perform the same function presently being performed by beacon assemblies. For example, the selective activation of one pole may provide an indication to the vehicle that it is supposed to take a particular path to a junction point. This is especially useful for certain types of systems which do not provide wireless destination codes to the vehicle. The present invention provides the ability for such system to locally indicate to the vehicle, for example, the beckoning of that vehicle by an operator or the like.
These and other objects, advantages and features of this invention will become apparent upon review of the following specification in conjunction with the drawings.